Minifocus X-rays using old valves/tubes

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By Leslie Wright

Many people, myself included, have built x-ray sources using common radio tubes run in cold-cathode mode. Such tubes include the PD-500, 2X2, 6EN4, and other assorted high voltage rectifiers, or shunt stabilizer tubes.

Whilst it is possible to take radiographs using these tubes, the resulting images are blurred, since the x-rays are produced over a wide target area, resulting in a large focal spot size. Commercial x-ray tubes, have comparatively small focal spots, and example being a small dental tube I have with a 3x1mm target area resulting in a virtual focal spot of 1x1mm. Other specialist tubes have even smaller focal spots, and are thus capable of incredible resolution.

However, as usual in the thermionic valve world, there are oddball tubes that show up. There is a particular shunt stabiliser Tetrode, the 6VS-1, that is capable of producing copious amounts of x-rays, from an incredibly small (sub millimetre) focal spot, that is capable of astounding resolution, and is easy to drive, and setup.

Above: The 6VS-1.

The key to this tubes remarkable properties, is the electrode structure.

Instead of a self shielding design, like the PD-500, the electrodes are planar, like one would expect to find in the electron gun of a CRT. This means that the anode surface is visible, AND, that it is possible to electrostatically focus the beam onto the anode, yielding a very small focal point.

Above: The 6VS-1 internal electrode structure.

The anode is the disc shaped piece of metal on the left, and is connected to the tubes top cap. If you look closely, you will see a small hole in the middle of the anode, where I pushed the tube too far beyond its ratings.

Above: A view of the anode with the tube running, note the white spot. This is the focal point in the tube, and is in fact red (the camera sees white, due to the IR emitted).

We can see that the maximum anode voltage is only 7.7kV at a beam current of 350 μA. In order to produce a useful amount of x-rays, the tube needs to be driven at at around 11+kV. However, so as not to melt the anode, the anode rating (in watts) shouldn’t be exceeded.

From the above data, we can calculate an anode dissipation of around 2.69W MAX. So to be on the safe side, we can call it 2W, we can then calculate the maximum current (with a little headroom) to be 66 μA at 30kV.

In order to power the tube the correct voltages need to be present on the remaining grids, and cathode. Since I didn’t want to go to all the bother of building a negative supply for the cathode, the “modulator” or grid 1 is held at ground potential, and the cathode, held 0-100v positive with respect to it (which is essentially the same thing). The “accelerator” grid 2 is held at +200V.

Since when the anode voltage is present, there will be significant charge on the tubes internal electrodes, no separate supply is actually needed for these, instead I clamped the grid and cathode to ground with zener diodes.

The 6VS-1 appeared on another site a while ago, but I have since lost the link. The grid and cathode were stablilised with neon lamps. The neons do essentially the same job as the Zeners in my design, however they are somewhat unstable, and the grid and cathode can swing sharply negative, resulting in unstable x-ray output.
Zeners were chosen for my design, along with smoothing caps, greatly enhancing the performance of the tube.

Above: Circuit diagram of the modifications to the 6VS-1.

The arrangement of the zeners can be seen in the diagram, above making the explanation a little clearer. The components were simply soldered directly to a tube base. R1 is used to control beam current, making altering the current a walk in the park. The higher the resistance of R1 the higher the cathode voltage, and thus the LOWER the beam current. Conversely the lower the resistance, the HIGHER the beam current.

In later designs, the zener value D1 was altered to ~130v, and D2 to ~260V, and the caps were omitted.

Above: A modified 6VS-1. Note the lead shield around the middle of the tube.

Using the tube:

In the above descripton 30kV was chosen as a ballpark figure for running the tube at. However, as stated, this is unlike any other vacuum tube you will come across!

The tube is capable of producing enough x-rays at just 11kV, to observe fluorescence in x-ray sensitive phosphors. It produces enough at 20kV to sucessfully radiograph objects in a resonable amount of time!

At around 25kV the tube begins to behave eratically. This is down to the gettering coating most of the inside of the tube. The high voltage charge “arcs” across it, causing the x-ray source to flicker somewhat. Whilst this phenomenon isn’t a problem for taking radiographs on film, it is irritating when using a fluoroscope, and it is highly likely that damage will eventually be done to the tube.

In practice, I run mine at between 11 and 22kV, at a maximum current of around 50µA. This is quite adequate, and at the same time I can be rest assured that I will get many hours of use out of the tube.
There is no need to limit the anode current by means of a ballast resistance, as shown on one of the above photos, since this is taken care of by the grid bias. The heater volage is provided my means of a small 6.3v transformer.

The tube mostly radiates x-rays 360 degrees around the middle of the tube, so I made a small lead shield, with a 10mm hole in it, to absorb all the unused x-rays. Even with the shield in place, there is significant leakage from the anode end of the tube, as a result of x-ray transmission through the anode, and secondary electron collisions on the tube walls. There is also leakage from the cathode end, as some x-rays inevitably make it through the metal electrode structures.

Above: A strip of fluoroscopy screen was wrapped around the middle of the tube. You can clearly see the fluorescence around the middle of the tube. The dark lines that cross the fluorescent area, are the electrode supports, in the way of the beam.

As well as the central shield, I strongly advise shielding the whole tube, in a lead lined box.
The tube will overheat if run at full current and voltage for too long, and should be avoided. 20 seconds at 20kV, and 75µA is about all it can handle, and requires a period of cooling before being run again.
Overheating generally isnt too catastrophic. When a tube overheats, its current draw will suddenly increase, and x-ray emission will cease. If this happens, all power (including heaters) is simply cut from the tube, and it is allowed to cool for a few minutes. If you are using a PSU capable of sourcing over 150µA, a fuse would be required to protect the tube.

Above: A piece of fluoroscopy screen was placed on the lead aperture. The beam that escapes is nice and circular.

Above: in this photo, a lighter was placed between the tube, and a piece of fluoroscopy screen. The image is a lot sharper in real life, but the camera has difficulty focussing on the dim image. It is best viewed in darkness.

These are fun little tubes, however I must stress, that they do produce copious amounts of radiation, so be sure to shield them well, and run a dosimetry program!

These modified tubes will shortly be available for sale through eBay, to fund the costs of this, and other ongoing projects.

Click here to see what I`m selling !

An exmple of the modified tubes I am selling.

The tubes will be provided fully tested, with the lead shield in place. The tube base is pre-wired with zeners, and a 100k pot.
The twisted blue and blue/white leads are the heater connections. The solid blue one should be connected to the 0V rail of your EHT supply. The top cap is the anode connection.

It will be up to the purchaser to source a 6.3v ac heater supply, and to build an EHT supply for it.
I will not provide step by step instructions on the building of a high voltage supply, simply because I feel that if you can’t design a suitable supply for it, then you really shouldn’t be messing with x-rays! 🙂

I accept no responsibility for injuries caused by using these tubes, or the information presented on the site. Safety is YOUR responsibility!

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